Cnt-pi complex having emi shielding effectiveness and method for producing the same

ABSTRACT

The present invention provides a complex including carbon nanotubes (CNT) and polyimide (PI), and a method for producing the same. The CNT-PI complex possesses good electromagnetic shielding effectiveness. The CNT-PI complex primarily includes polyimide and carbon nanotubes dispersed in the polyimide. The method for producing the CNT-PI complex first disperses carbon nanotubes in a solvent by adding a dispersant and using an ultrasonic oscillator. Then the carbon nanotubes dispersion is mixed with polyamic acid to give a CNT-PI dispersion. The CNT-PI dispersion is then dried to form a film or layer of the CNT-PI complex. The dispersant used in this invention is an ionic liquid including organic cations and inorganic anions. The produced CNT-PI complex presents better networked structures and electrical conductivity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a CNT-PI (carbon nanotubes-polyimide)complex and a method for producing the same, and particularly to aCNT-PI complex having good electromagnetic interference (EMI) shieldingeffectiveness. The method for producing the complex includes dispersingcarbon nanotubes, mixing with polyamic acid which is then transferred topolyimide by thermal imidization at 100˜360 degree C.

2. Related Prior Art

Currently, applications of materials used for EMI shielding can beclassified into two types. One is to deposit the material havingelectrical or magnetic conductivity on a substrate. The other is to mixor fill the material having electrical or magnetic conductivity with orin a substrate. Accordingly, electromagnetic waves can be reflected orabsorbed by such materials without passing through.

Carbon nanotubes as a material having the above properties have beenapplied to EMI shielding. For example, in “The Electromagneticinterference of Multi-Walled Carbon Nanotubes-Polymer Composite”, 2006,Hong Chien-Fu mentioned an application of carbon nanotubes mixed inepoxy to EMI shielding. The results indicated that the EMI shieldingeffectiveness at 1 GHz was only 1.6 dB when 5 wt % of carbon nanotubeswas present, which was not satisfactory for practical use. U.S. Pat. No.7,413,474 mentioned a material having EMI shielding effectiveness whichcontained carbon nanotubes and polymers such as polyethyleneterephthalate (PET), polycarbonate (PC), acrylonitrile butadiene styrene(ABS) and a mixture of PC/ABS. However, no data was disclosed to showtheir EMI shielding effectiveness.

In addition, how to uniformly disperse carbon nanotubes in a polymermatrix is important. So far, carbon nanotubes can reach the maximumconcentration at about 10 wt % but has poor electrical conductivity.

To solve the above problems, the present invention develops a complexmade from carbon nanotubes and polymers and having good EMI shieldingeffectiveness.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a CNT-PI (carbonnanotubes-polyimide) complex and a method for producing the same, sothat the CNT-PI complex has good EMI (electromagnetic interference)shielding effectiveness.

To achieve the above object, the CNT-PI complex of the present inventionprimarily includes polyimide and carbon nanotubes dispersed in thepolyimide. The CNT-PI complex has a thickness of about 850˜10,000 μm andthe carbon nanotubes are present in the form of networks in thepolyimide. Additionally, the concentration of the carbon nanotubes inthe complex is about 10˜50 wt %, and preferably about 25˜30 wt %. Oneindividual carbon nanotube has a diameter of about 30˜60 nm and anelectrical conductivity of about 10⁻²˜10⁻⁵Ω·cm. The complex has anelectrical conductivity of about 10⁻⁴˜10¹ (S/cm).

The method for producing the CNT-PI complex primarily includes steps:(1) dissolving a dispersant in a solvent and dispersing carbon nanotubesin the solvent containing the dispersant by a magnetic stirrer,ultrasonic vibration or mechanical blending to form a dispersion of thecarbon nanotubes, wherein the dispersant is an ionic liquid containingorganic cations and inorganic anions; (2) mixing the dispersion of thecarbon nanotubes of step (1) with polyamic acid, precursor of polyimide(PI), to form a suspension of CNT and polyamic acid; and (3) thermalimidizating the suspension of step (2) to form a CNT-PI complex having adesired thickness.

In the above step (1), the dispersant is an ionic liquid includingorganic cations and inorganic anions. The organic cations can be amine,phosphorous, sulfide, pyridine or imidazolium and the inorganic anionscan be BF₄ ⁻, PF₆ ⁻, SbF₆ ⁻, NO₃ ⁻, CF₃SO₃ ⁻, (CF₃SO₃)₂N⁻, ArSO₃ ⁻,CF₃CO₂ ⁻, CH₃CO₂ ⁻ or Al₂Cl₇. Examples of the dispersant includetriethylamine hydrochloride (TEAC), 1-hexadecyl-3-methylimidazoliumchloride (HDMIC), dihexadecyl dimethylammonium bromide (DHDDMAB),tributyl hexadecyl phosphonium bromide (TBHDBP), etc. The dispersant inthe solvent has a concentration of about 0.1˜5 wt %. The solvent can beN-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF), dimethyl formamide(DMF), dimethyl acetamide (DMAC) or toluene. The carbon nanotubes in thedispersion has a concentration of about 5˜15 wt %. In the step (1), thecarbon nanotubes can be dispersed into the solvent containing thedispersant preferably by ultrasonic vibration.

In the above step (2), the polyamic acid has a concentration about 10˜20wt %. The polyamic acid can be previously dissolved in a solvent thesame as that of step (1). The dispersion of the carbon nanotubes can bemixed with the polyamic acid by a blender and an ultrasonic vibrator.

In the above step (3), the temperature for thermal imidization is about100˜365° C. The suspensions of CNT and polyamic acid can be previouslycoated on a substrate and then heated so that the solvent can be removedand then the polyamic acid is transferred to polyimide to achieve theCNT-PI film. A plurality of the films can be further pressed at a propertemperature to obtain a combinative film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the CNT-PI suspensions with different contents of carbonnanotubes.

FIG. 2 shows the CNT-PI thin film.

FIG. 3 shows electrical conductivity of the CNT-PI thin film withdifferent contents of carbon nanotubes.

FIG. 4 shows the SEM image of the CNT-PI thin film.

FIG. 5 shows relationship between the thicknesses of the CNT-PI thinfilm and EMI shielding effectiveness (SE) thereof.

FIGS. 6 and 7 show the far-field and near-field EMI shieldingeffectiveness (SE) of the combinative film.

ATTACHMENT 1 shows the dispersing statuses of the ionic liquids insolvents.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 1. Selecting theDispersant and the Solvent

Prepare four ionic liquids (IL) respectively from triethylaminehydrochloride (TEAC), 1-hexadecyl-3-methylimidazolium chloride (HDMIC),dihexadecyl dimethylammonium bromide (DHDDMAB) and tributyl hexadecylphosphonium bromide (TBHDBP). Equal amounts of carbon nanotubes arerespectively added into the above ionic liquids to form four IL-CNTmixtures. Each of the IL-CNT mixtures is separately mixed with solventsN-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF) and toluene toobtain solutions containing 15 wt % of carbon nanotubes. The carbonnanotubes used in the preferred embodiments of the present inventionhave a diameter about 30˜60 nm and an electrical conductivity about10⁻²˜10⁻⁵Ω·cm.

Chemical structures of the ions in the above ionic liquids can influencethe uniformity and stability of the carbon nanotubes dispersed in thesolvents, for example, lengths of side chains, one-arm or two-arm, etc.As shown in ATTACHMENT 1, the carbon nanotubes in TBHDPB perform thebest in terms of uniformity. The carbon nanotubes in THF and tolueneprovide good uniformity and stability even for 12 hours. The carbonnanotubes in NMP also provide good stability for 4 hours. The ionicliquid HDMIC can maintain good stability for 12 hours in THF, and 20minutes in NMP and toluene. Other ionic liquids such as TEAC havingshort arms and DHDDMAB having one arm can maintain good stability for2˜3 hours in NMP.

As NMP is more commonly and frequently used in industries, and thusselected as a solvent in the preferred embodiments of the presentinvention. HDMIC provides similar dispersion effect as TBHDPB and isselected as a dispersant in the preferred embodiments as HDMIC containsnitrogen which is close to polyimide in structure.

2. Preparing the Suspensions of CNT and Polyamic Acid

HDMIC (1 wt %) is dissolved in NMP, and then carbon nanotubes aredispersed therein by ultrasonic vibration to form three dispersionsrespectively containing 10 wt %, 20 wt % and 30 wt % of carbonnanotubes. By means of a blender (2000 rpm) and an ultrasonic vibrator(40 Hz), the above dispersions containing different concentrations ofcarbon nanotubes are separately mixed with polyamic acid (16 wt %,previously dissolved in NMP) to form CNT-PI suspensions. Polyamic acidis the precursor of polyimide. FIG. 1 shows the result.

3. Preparing the CNT-PI Film by Thermal Imidization

The above suspensions of CNT and polyamic acid are separately coated onglass substrates (210×97 mm) and then placed in an oven (100˜360° C.).The solvent is removed and then the polyamic acid is transferred topolyimide to achieve black thin CNT-PI films with thicknesses ranging20˜30 μm, as shown in FIG. 2. Finally, forty thin films are pressed at aproper temperature to obtain a combinative film having a thickness of800˜1000 μm.

Analysis and Test 1. Electrical Conductivity

Surface electrical resistance or conductivity of the thin film having athickness of about 10˜20 μm are measured by means of four-point probe.The results are shown in FIG. 3, in which electrical conductivityincrease with contents of carbon nanotubes. Electrical conductivity ofthe thin film can reach to 10¹ S/cm when the concentration of the carbonnanotubes in the thin film is 30 wt %. Conventionally, 50 wt % of carbonnanotubes is needed to reach 10¹ S/cm of electrical conductivity becauseof poor dispersion. In the present invention, the carbon nanotubes canbe dispersed well, and therefore 30 wt % is enough to reach 10¹ S/cm ofelectrical conductivity.

2. SEM Analysis

The CNT-PI complex of the present invention is observed by means ofscanning electron microscope (SEM). As shown in FIG. 4, the thin filmcontains the carbon nanotubes loosely dispersed in polyimide when itcontains 10 wt % of the nanotubes. Apparently, when the thin filmcontains 30 wt % of the carbon nanotubes, networks of carbon nanotubescan be observed. In other words, contents of the carbon nanotubes inpolyimide can influence forms of the carbon nanotubes in the thin film.Theoretically, when more complete networks of the carbon nanotubes arepresent, electrical conductivity thereof will increase, as shown in theabove measurements.

3. EMI Shielding Effect

A. Relationship between thickness of the CNT-PI thin film and EMIshielding effectiveness

FIG. 5 shows relationship between the thickness of the CNT-PI thin filmof the present invention and EMI shielding effectiveness (SE). Theresults indicate that EMI shielding effectiveness of the thin filmincreases with its thickness. However, EMI shielding effectivenessbecomes acceptable when the thickness of the CNT-PI thin film of thepresent invention is more than 850 μm. Optimal EMI shieldingeffectiveness is achieved when the content of the carbon nanotubes is 30wt %. Therefore, the following measurements are made with thin filmshaving a thickness of 850 μm.

B. Far-Field

According to ASTM D4935, far-field EMI shielding effectiveness of thecombinative film is measured. As shown in FIG. 6, far-field EMIshielding effectiveness (SE) of the combinative CNT-PI film of thepresent invention can reach 40˜45 dB at 1˜3 GHz.

C. Near-Field

In a laboratory without electromagnetic reflection, a monopole antennais used as a radiation source. Radiation values of the monopole antennaare measured before and after the combinative film is applied.Difference in the values indicates near-field EMI shieldingeffectiveness. As shown in FIG. 7, near-field EMI shieldingeffectiveness (SE) can reach about 37˜42 dB at 2.5˜3 GHz.

4. Eye Mask Margin

According to the SONET OC-48 specification, a monopole antenna is usedas an interference source to measure the eye mask margin of an opticalreceiver module (2.5 Gb/s). The eye mask margin of the optical receivermodule shielded with the combinative film indicate that the eye maskmargin of the optical receiver module (2.5 Gb/s) increases from 43% to56% after the combinative film (having a content of carbon nanotubes of30 wt % and a thickness of 850 μm) is applied. In other words, thecombinative film can effectively shield and block the optical receivermodule (2.5 Gb/s) from outside EMI.

According to the above, the present invention indeed provides a CNT-PIcomplex having good EMI shielding effectiveness. The CNT-PI complexpresents a better network form and better electrical conductivity, sothat EMI shielding effectiveness can be promoted. The CNT-PI complex canbe applied to non-metalic and low-resistance flexible substrates, forexample, resins and thin films.

1. A CNT-PI (carbon nanotubes-polyimide) complex having EMI(electromagnetic interference) shielding effectiveness, wherein theCNT-PI complex has a thickness of about 850˜10,000 μm and comprisespolyimide and carbon nanotubes dispersed in the polyimide in networks.2. The CNT-PI complex of claim 1, which contains about 10˜50 wt % of thecarbon nanotubes.
 3. The CNT-PI complex of claim 1, which contains about25˜35 wt % of the carbon nanotubes.
 4. The CNT-PI complex of claim 1,wherein the carbon nanotubes have a diameter of about 30˜60 nm
 5. TheCNT-PI complex of claim 1, wherein the individual carbon nanotube has anelectrical conductivity of about 10⁻²˜10⁻⁵Ω·cm.
 6. The CNT-PI complex ofclaim 1, which has an electrical conductivity of about 10⁻⁴˜10¹ (S/cm).7. A method for producing a CNT-PI complex having EMI shieldingeffectiveness, comprising steps of: (1) dissolving a dispersant in asolvent and then dispersing carbon nanotubes (CNT) in the solventcontaining the dispersant by magnetic stirrer, ultrasonic vibration ormechanically blending to form a dispersion of CNT, wherein thedispersant is an ionic liquid containing organic cations and inorganicanions; (2) mixing the dispersion of the carbon nanotubes of step (1)with polyamic acid to form a suspension of CNT and polyamic acid; (3)thermal imidizing the suspension of step (2) to form a CNT-PI (carbonnanotubes-polyimide) complex having a thickness of about 850˜10,000 μm.8. The method of claim 7, wherein the organic cation of the dispersantof step (1) is amine, phosphorous, sulfide, pyridine or imidazolium. 9.The method of claim 7, wherein the inorganic anions of the dispersantsof the step (1) is BF₄ ⁻, P F₆ ⁻, SbF₆ ⁻, NO₃ ⁻, CF₃SO₃ ⁻, CF₃SO₃)₂N⁻,ArSO₃ ⁻, CF₃CO₂ ⁻, CH₃CO₂ ⁻ or Al₂Cl₇.
 10. The method of claim 7,wherein the dispersant of the step (1) is triethylamine hydrochloride(TEAC), 1-hexadecyl-3-methylimidazolium chloride (HDMIC), dihexadecyldimethylammonium bromide (DHDDMAB) or tributyl hexadecyl phosphoniumbromide (TBHDBP).
 11. The method of claim 7, wherein the dispersant ofthe step (1) has a concentration of about 0.1˜5 wt % in the solvent. 12.The method of claim 7, wherein the solvent of the step (1) isN-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF), dimethyl formamide(DMF), dimethyl acetamide (DMAC) or toluene.
 13. The method of claim 7,wherein the carbon nanotubes is dispersed in the solvent containing thedispersant of the step (1) by ultrasonic vibration.
 14. The method ofclaim 7, wherein the carbon nanotubes of the step (1) has aconcentration about 5˜15 wt % in the dispersion.
 15. The method of claim7, wherein the dispersion of the carbon nanotubes and the polyamic acidof the step (2) are mixed by a blender and an ultrasonic vibrator. 16.The method of claim 7, wherein the polyamic acid of the step (2) is asolution having a concentration about 10˜20 wt %.
 17. The method ofclaim 7, wherein the polyamic acid of the step (2) is previouslydissolved in a solvent the same as that of the step (1).
 18. The methodof claim 7, wherein the step (3) is controlled at about 100˜365° C. forthermal imidization.
 19. The method of claim 7, wherein the CNT-PIcomplex of the step (3) contains about 10˜50 wt % of the carbonnanotubes.
 20. The method of claim 7, wherein the CNT-PI complex of thestep (3) contains about 30 wt % of the carbon nanotubes.
 21. The methodof claim 7, wherein the CNT-PI complex of the step (3) has an electricalconductivity of about 10⁻⁴˜10¹ (S/cm).